The cross in its modern form was first used by the Chaldeans and the Greeks in the sixth century B.C. Modified forms of the cross were also used by Hindus, Buddhists and Egyptians as far back as 3000 B.C. Christians started to widely use the cross as a symbol in the second and third centuries A.D.
Chaldean crosses evolved from a circular symbol representing the Sun. The two beams that form the cross were then added to represent the Sun's beams. These crosses symbolized the Chaldean sun god Tammuz. The Greek gods Diana and Bacchus were sometimes depicted with similar crosses.
The religious symbol of ancient Hindus and Buddhists was the swastika, an equilateral cross with bent ends. Hittites and North Africans also used the symbol. The Egyptians used the symbol of the ankh, a cross with a circle replacing the top end. The ankh was usually considered a representation of life but was also often a part of depictions of the solar deity, Sekhet.
One of the earliest depictions of a cross associated with Christians is the Alexamenos graffito, which dates back to the second century A.D. However, Christians did not fully adopt the cross as a symbol until the third century, when the emperor Constantine, who was already using the symbol in war, adopted the Christian religion. Christian artists started to depict Jesus on a cross in the sixth century A.D.